^SnL. aS^mS'] of Ohjed-glasses for the Blicroseoj^e. 95 
be observed, as described in prism-work, shown by the following 
examples. Fig. 3 represents a plano-convex lens which has been 
made and finished upon a flat disc of 3 
glass, to which it has been attached 
with hard Canada balsam. The two 
discs are cemented to the stick with 
black sealing-wax ; the lens and sup- 
porting disc are rough ground on the 
zinc plate till they nearly fit the concave gauge, they are then 
ground in the brass mould till the lens measures very nearly the 
diameter required, leaving a small allowance for smoothing and 
polishing. 
For double convex lenses, the disc of glass, cemented on a stick 
as usual, is first ground and polished on one side. A piece of 
glass tube of suitable size is selected for a handle, and the end of 
the bore ground out to a similar radius ; the polished side of the 
unfinished lens is then cemented ^ 
into this concavity, and the lens 
and tube ground and polished off 
together, as shown by Fig. 4, taking 
the same precautions as before to 
work the lens up to the exact 
diameter required. The dotted lines show the rough disc as 
cemented down. By this method all the marginal errors are taken 
up by the glass tube-holder, of which an assortment of various sizes 
will be required, from a minute bugle, up to half-an-inch or more 
in diameter. Before using the holders again for other lenses, the 
end must be ground out on each occasion, so as to increase the 
diameter of the cup. The lens, when taken out by being warmed, 
will have a knife-edge perfect to the extreme. 
In minute lenses, some difiiculty will be experienced in obtaining 
the measurements by means of gauge instruments, when near the 
right diameter. I therefore, for small sizes, always use the micro- 
scope with micrometer eye-piece, having previously taken the exact 
size from the diameter of the cell in which the lens is to go. This 
is very accurate and convenient. After the finished lens is taken 
out of the holder, if it should be found too large to enter the cell, it 
may be slightly cemented to the end of a wire, and twisted into a 
piece of the finest emery-paper, held in a hollow form, and the keen 
edge is taken off till it passes through. 
The single fronts for the highest powers, from their form, do 
not admit of being ground in this way. A piece of brass or steel 
is screwed into the mandril, and the end turned of a size to enter 
the cell into which the lens is to go ; the end is turned flat, or 
rather slightly hollow, and the centre taken out. A piece of crown- 
glass is cemented by its pohshed side to the flat end, with the best 
